High-frequency generator



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March 30, 1948. P- D, ZOTTU 2,438,595

HIGH-FREQUENCY GENERATOR Filed May 3, 1944 9 Sheets-Sheet 2 IN V EN TOR.

BY PAUL D. 20T-ru Owl-o W Filed lay 3, 1944 9 Sheets-Sheet 3 w a al $5/ az) ma 85.1! 84m Af FIG.. 5 95 IN V EN TOR.

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March 30, 1948.

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PAUL D. 2mm 05d? www AH'orngy March 30, 1948. P DI ZAOTTU 2,438,595

HIGH-FREQUENCY GENERATOR Filed May 3, 1944 9 Sheets-Sheet 5 INVENTOR. BY PAUL D. ZOTTU AHOmey.

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HIGH- FREQUENCY GENERATOR Filed May 3, 1944 9 Sheets-Sheet 6 FIG. 8

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56.9 BY PAUL D, Zorru 0h71) wwdwl ATTORN EY Filed May 3, 1944 9 Sheets-Sheet 7 INVENTOR. BY PAUL D. ZOTTU d @www Attorney March 30, 1948.

P. D. ZOTTU HIGH-FREQUENCY GENERATOR Filed May 5, 1944 9 Sheets-Sheet 8 Fl@ H I N ly 'EN TOR. PAUL D. ZOTTU.

BY@ l) pmMW/ @Hm-nef March 30, 1948. P. D. zoTTU HIGH-FREQUENCY GENERATOR Filed May 3, 1944 9 Sheets-Sheet 9 kvm..

INVENTOR. PAUL D. ZOTTU4 7J? wmm@ Mics-ned,

Patented Mar. 30, 1948 HIGH-FREQUENCY GENERATOR Paul D. Zottu, Indian Hills, Ky., assignor to The Girdler Corporation, Louisville, Ky., a. corporation of Delaware Application May 3, 1944, Serial No. 533,997

(Cl. Z50-36) 12 Claims. 1

This invention relates to high frequency generators, more particularly to systems and apparatus of the type adapted for the generation of high frequency with large power output capabilities, and has for an object the provision of such a system which combines mechanical ruggedness with electrical simplicity and relatively high eilciency.

For many years it has been recognized by those skilled in the art that dielectrics, materials both liquid and solid of a character generally considered as insulators, may be heated if subjected to an electric field of substantial potential and of relatively high frequency. From the time of Teslas first observation of the internal heating of the body and of dielectrics, by electrical current of high frequency, apparatus of different kinds have been employed to produce the heating effect. High frequency energy comprising damped oscillations from spark gaps, as well as continuous wave oscillations from apparatus of the type used for radio broadcasting, have been utilized. The internal or diathennic heating has been applied to materials of widely differing a single stage oscillator which is not only eflicient but which may be readily adjusted to meet the requirements of a wide variety of industrial operating conditions.

In carrying out the invention in one form thereof, there is provided a high frequency gen-- erator having the desirable characteristics of an amplifier and a power output which is satisfactorily high as compared with the input. The invention includes a method of introducing into the output circuit loads of widely varying electrical constants, which constants may vary continuously during the application of high frequency energy. 'I'his is accomplished by the provision of a circuit and circuit elements which provide a continuously variable transformation ratio so that, regardless of the nature of the load in the high frequency secondary circuit, the primary circuit can be madegtq appear resistive and essentiallncf. -consta n t.-1g ni"f over, the adjustments m beuair'lfed'it' with argeaafiuiiis'e@garantierte como circuit. 'lhuswthek generator. may,n be"'o5era ted efficientltLalLhighpcnirenlevels..evenN though the character; it has been applied to solutions, to 25 electrical characteristics 0f the ,materiaelg all parts of the human body, to various animals. heated vary to a substantial degreeduring the to grains, cereals, and other organic and inorheating p peratig ganic no-n-conducting substances of widely diftering character such as plastics, glass, plywood, and also to conducting and semi-conducting substances.

Though the application 0f high frequency heating to dielectric materials has been extensive, much is yet to be desired in the way of systems and apparatus of relatively low initial, maintenance, and operating costs for producing the Further, in accordance with the invention there is provided a single stage oscillator of th 1 push-pull type, which is neutralized, preferabl .l by means of criss-cross balancing condensers be-f tween the anodes and grids of the tubes. TankI circuits may be provided in both the input and.' output circuits. These tank circuits are inductively coupled together by means of pivotally lmounted coils through which the degree of feed back coupling may be changed through relatively f'wide limits. The grids of the tubes may be pro- "1` lvided with a negative bias adequate to insure the ideslred operating characteristics. Additionally,

g, provision is made to prevent the appearance of required high frequency energy. Resort has been had to coupled stages consisting of a master oscillator. a buffer amplifier and a final power amplifier stage. Such multistage systems are costly both in first cost and in maintenance and are relatively complicated in operation.

It is an object of the present invention to provide a single stage high frequency generatingf` ;'system which has all of the advantages of the 4 l'multistage system and few of the disadvantages. 1 and which is particularly suited for high power,

or relatively high power, industrial installations, the umm? con and through the mner coliductor for high frequency electric heating. of a coaXlal transmissionline, through which the It is a further object of the invention to pro- 5 high-frequency energy 1S Supplied to the load Vide an eflicient system having a power output circult' adequate to meer the heating requirements of Numerous addliional advantages and features such industrial installations, and which at the of the invention are set forth in the detailed desame time is simple and economical to operate SCIDOD based 11D0I1 the aCCOmDanyng draW- and requires minimum upkeep costs. 55 ings, in which:

Another object of the invention is to provide Fig. 1 is a schematic Wiring diagram of the i. l l.

l, bothersome parasitics in the various meshes of ii the circuit. Provision is also made for the circulation of a cooling fluid through the inductor l lin the output tank circuit, as well as through the iinductor forming the secondary or output coil,

principal circuits of an oscillator embodying the invention;

Fig. 2 is a plan View of the principal component parts of a typical apparatus c-omprising mechanical features of the oscillator of Fig. 1;

Fig. 3 is an end view illustrating the sub-assembly comprising the output coupling coil, the output tank inductor and the feedback coil;

Fig. 4 is a fractional end view of the pivotal support for the output coil;

Fig. 5 is a fractional end view of the pivotal support for the feedback coil;

Fig. 6 is a view in perspective of the input tank circuit together with the exciting coil therefor;

Fig. '1 is a wiring diagram of a modified oscillator or high frequency generator;

Figs. 8 and 9 are wiring diagrams of modified forms of oscillators or high frequency generators: and

Figs. 10, 11 and 12 are fractional end views of modified forms of pivotal supports for certain of the coils, such, for example, as the feedback coils.

Referring to the drawings, the invention in one form is illustrated in Fig. 1 as comprising a pair of electric valves I and II connected in a circuit which, in many respects, is similar to a conventional push-pull oscillator. The anodes of the tubes I0 and I I are respectively connected to a divided output inductor comprising sections I2 and I3. The output tank capacitor I4 is connected in shunt or parallel relation with the inductor I2-I3. A suitable source of anode potential is indicated by the terminals marked B+ and B-. The connection from the B+ terminal leading to the midpoint of the inductor I2-I3 includes a high frequency choke coil I4a. The respective control grids of the tubes I0 and II are connected to spaced points on the inductor I of the input tank circuit, which also includes a capacitor I6. The control grid of the tube I0 has in circuit therewith a parasitic suppressing resistor I1, the connection to the inductor I5 being completed by the contactor I8. Similarly. the control grid of the tube II has in circuit therewith a parasitic-suppressing resistor 20 with a contactor 2I completing the connection to the inductor I5. The respective grid circuits are completed by way of conductors 22 and 23, resistors 24 and 25, a milliammeter 26, a grid bias battery C, conductors 21, 28 and 29, and thence to the midpoint of the secondary windings of the respective filament transformers 3I and 32.

As well understood in the art, there exists between the anodes and the grids of each of the valves or tubes I0 and II a substantial capacitance, substantial in the sense that there is a feeding back or capacitive return to the grid circuit of some of the signal energy appearing in the output circuit. The effect of the anode-grid capacity on each tube is neutralized by means of the criss-cross capacitors 34 and 35 which, it will be observed, are respectively connected from the anode of one tube to the grid of the opposite tube. The neutralizing capacitors 34 and 35 are of a size, or variable in manner, such that the signals fed back through these capacitors to each grid circuit have a magnitude equal to that of the signals fed back through the anode-grid capacity. Since this neutralizing voltage for each tube is derived from the opposite tube of the push-pull combination, itis 180 out of phase with the voltage which is fed back through `the anode-grid capacity and hence neutralizes the effect of the latter.

'I'he respective input or grid circuits have in shunt therewith resistors 36 and 31 which provide a better regulated load for the input circuits. The mid-point of these resistors is also connected to the conductor 22. The resistors 36 and 31 may be omitted in some oscillators, but if certain parasitics develop these resistors eliectively suppress them.

In accordance with the invention, a divided feedback coil, comprising coil sections 4D and 4I, is inductively associated with the output tank inductor I2-I3. The feedback coil applies some of the output energy to the divided coil sections 42 and 43. In terms of the output of the oscillator as a whole, the energy fed back to the input circuit is relatively small. It is only enough properly to energize the input tank circuit comprisng the inductor I5 and the capacitor I6.

The load circuit is coupled to the output tank inductor I2I3 by means of an output coil 45. The load circuit also includes a variable tuning coil 46. As shown, the load may consist of plywood bundles 41 and 48, located between the platens 49 and 5I! of a press. A central electrode or conducting plate 5I is connected directly to the tuning coil 46. This arrangement provides a return circuit which divides between the plywood bundles 41 and 48. The platens 49 and 50 are connected together as indicated by conductor 52, the return circuit being completed by way of conductor 53. For a more detailed disclosure of the electrode arrangement in the press, reference may be had to my Patent No. 2,307,344, dated January 5, 1943.

In a push-pull oscillator several mesh circuits are present. If there are present in any mesh or loop circuit inductive and capacitive reactance, such circuit will exhibit characteristics of resonance at a particular frequency. If energy is applied to such a circuit, oscillations will occur at that particular frequency. 'Ihese spurious oscillations at frequencies other than the desired frequency are known as parasitics. Specically, one parasitic oscillation may arise by a tendency of the tubes to act as though they were connected in push-pull with condenser I4, effectively short-circuiting coils I2 and I3. The frequency of this parasitic may be of the order of ten times the desired frequency. However, in accordance with the invention, it is eifectively suppressed by the resistors I1 and 20 connected in series in the respective grid circuits. The resistance necessary for suppression of parasitics of this type is low and may be of the order of only a few ohms. Additionally to suppress the foregoing parasitic and additional parasitics, each side of each filament is connected through resistors 54 and 55 and capacitors 56 and 51 to ground, the common circuit including the conductor 29. These circuits, including both resistance and capacitance, bypass the parasitics through circuits of high loss, thus suppressing same. If desired, these by-pass circuits may be replaced by resistors connected directly in series with each side of each filament. However, the by-pass circuits are preferred. The choke coil I4a in the anode circuit also functions to dampen or suppress parasitics.

There sometimes appears in the grid-cathode circuit a parasitic having a frequency within the audio range. 'I'his parasitic is effectively suppressed by means of the resistor 24. The resistor 25 and the capacitor 6I) comprise respectively the grid leak resistor and capacitor and provide in manner well understood by those skilled in the art a negative bias on the respective control grids. The C battery provides a. minimum negative bias for the respective grids during stand-by periods.

The criss-cross capacitors 34 and 35 neutralize, as has already been explained, the gridanode capacity of each tube and they respectively overcome the effect of unwanted feed back of energy from the output to the input circuit. In terms of operation this means that if the feed back coil 40-4I is moved out of inductive relation with the inductor |2-I3 the oscillator will stop oscillating. Since there will not be a return of energy from the output to the input circuit when the feedback coils are in neutral or noninductive relation with the inductor |2-l3, the oscillator will not oscillate. In this respect the oscillator possesses one desirable characteristic of an amplifier. The advantage of insuring that oscillations will not be generated upon movement of the feedback coil to a neutral position is that the oscillator is quiescent before application of the load thereto. On application of the load the combined oscillator-amplifier can be made to generate oscillations of proper amplitude, thus simplifying the problem of loading and operating the same. In accordance with the present invention the oscillator or high frequency generator is at all times under control and at all times produces high frequency energy at the desired frequency, and with the desired amplitude, and in the substantial absence of parasitics of any consequential magnitude either before or during application of the load.

The present invention differs from push-pull oscillators of the prior art in that the neutralizing capacitors 34 and 35 feed back energy in manner to overcome the capacity feedback through the interelectrode capacity of the tubes and while at the same time the feedback coil 40-4I is arranged inductively to return a portion of the output energy to the tank circuit comprising the inductor I5 and the capacitor IS. These provisions, together with output and input tank circuits of relatively large current-carrying capacities and of high electrical eiciency, stabilize the frequency to an extraordinary degree. Relatively wide changes in the electrical constants of the load circuit do not tend to change appreciably the frequency at which the oscillator as a whole is intended to operate.

If desired, the feedback circuit may be coupled to the output circuit by means of a series tuned circuit instead of being conductively coupled through the closed switches 51 and 58. By opening these switches the capacitors 59 and 50a are connected in series in the feedback circuit.

The operation of the oscillator as a whole, as applied to the heating of the plywood bundles 41 and 48, will be described following the discussion of combined electrical and mechanical features.

Referring to Figs. 2-4, the output coupling coil 45 is shown as comprising two turns of pipe or tubing, preferably copper, with the respective ends of the coil supported from a pair of pipes 45a and 45h which terminate in L-flttings 59 and 60, into which are secured additional lengths of pipe 6| and 62. These additional lengths of pipe are provided with bearings 63 and 64 as shown in Fig. 4. From one of the short sections of pipe there extends an operating arm 65 which is welded or otherwise formed integrally therewith. Resilient contact fingers 66 and 61 extend from each bearing member 63 and 84 into sliding and wiping engagement with the pipes 6| and 62. In this manner good electrical contact is established to the bearings, from which the further electrical connections are made. Thus, from bearing 63 one connection is completed by a sheet of copper 1U, connected to ground and to the steel supporting frame F. This connection corresponds to the one in Fig. l where one side of the coil 45 Ais shown directly Vconnected to ground G. The other connection from coil 45 is by way of a tubular or pipe conductor 68 which is fluid-flow connected with the conductor 52. Fig. 4. The respective bearings 53 and 64 are mounted on electrical insulating blocks 12 and 13 which may be of hardwood and bolted to the channel iron frame F.

In order to rotate the coil 45 from the position shown in Fig. 3 in a clockwise direction about the pivotal axis and toward an electrically coupled position with respect to the coil |2-l3, the operating arm 65 is connected by a link 15, Fig. 3, to a disc 18 which may be driven by any suitable means such as by the electric motor 11. For example, as the disc 18 is rotated in a clockwise direction, as viewed in Fig. 3, the link 15 rotates the arm in a clockwise direction to move the output coil 45 into a position of inductive coupling with respect to the tank coil |2-I3.

Though additional turns may be provided, each of the sections 40 and 4| of the feedback coil is shown in Fig. 2 as comprising a single turn, the respective ends of which terminate in L-shaped fittings into which there are threaded relatively short lengths of pipe 8U and 8|. The corresponding pipe, or tubular conductor 80, from each coil has a second L-tting on the end thereof, into each of which there extends one end of a horizontal pipe or tubular conductor 82. As best shown in Fig. 5, the horizontal conductor 82 is journalled as at 83 and 84.

Attached to the horizontal pipe or conductor 82 is an operating arm 85, the free end of which is connected to a link 95 for rotation of the conductor 82. In Fig. 5 the coils 40 and 4I have been diagrammatically illustrated above their respective supporting pipes 88 and 8|. Thus, it will be observed that the right hand side of coil 48, as viewed in Fig. 5, is connected by the pipeconductors 80, 82 and 80 to the left hand side of coil 4|. The right hand side of coil 4| is connected to the pipe-conductor 8|, which in turn is fastened to an arcuate conducting member 81, Fig. 3, carried by an insulating member 88 which in turn is secured to the pipe-conductor 82. The conducting segment 81 has cooperating with it a plurality of resilient contact ngers 89 which are carried by a tubular conductor 83. Similarly, the left hand side of coil 4G. as viewed in Fig. 5, is connected to and supported by the pipe conductor 8|, which is fastened to a similar arcuate conducting member carried by an insulating segment 9|, which is in turn secured to and carried by the conductor 82. Resilient contact members 92 complete the connection from the left hand side of coil 40 to a conductor 80, as viewed in Fig. 5. Thus the conductors and 93 though stationary, are effectively connected to the coils 40 and 4|. The arrangement is such that these coils inductively assist each other. That is, the voltage induced in them is cumulative.

As shown in Fig. 3, the free end of the operating arm 85 is connected to one end of a link 95 with the opposite end thereof connected to a disc 96 driven by a motor (not shown). Upon clockwise rotation of the disc 96 as viewed in Fig. 3 the feedback coil comprising the two sections 40 and 4| is moved in a counter-clockwise direction and into inductively coupled relation with the tank coil |2|3. It will now be seen that the horlzontal conductor 82 not only serves to support the feed back coils 40 and 4| but it also serves electrically to connect the end of one coil with the end of the other coil. The conductor 82 also provides the pivotal support about whose axis the coils 40 and 4| may be rotated. Conductor 82 also supports the V-shaped insulating members 88 and 9| which carry the arcuate conducting members through which the opposite ends of the coils are connected to the conductors 90 and 93.

Energy induced in the feed back coil 404I flows by way of conductors IOI and |02, Fig.`2, to the input tank circuit. It will be observed that these conductors are also tubular and are respectively connected to the conductors 90 and 93. For example, as shown in Figs. 2 and 5, the conductor by an L-tting is connected to a vertical conductor |04 which is threaded into an L- tting, Fig. 5. The connection from the conductor |02 into the conductor 93 is made in like manner by vertical conductor |03.

Cooling water may be circulated through the output coupling coil 45, as by the rubber or insulating hose connection indicated at |06 and the outlet tubular conductor 68, which leads, Fig. 2, to a second tubular conductor |0|. From inlet |06, the flow is through the tubular conductor 6I, Fig. 4, forming the left hand horizontal support, through the vertical conductor 45a, the tubular coil 45, the other vertical conductor 45h and thence through the other horizontal pipe 62 to the outlet pipe 68 and to conductor |01. As diagrammatically shown in Fig. 2, the conductor |01 leads to a tuning coil or inductor |0`|a, thence through a concentric line |01b to the platen 5| of the press. The huid outlet is taken from near the end of the inner conductor of the line |0'la as by insulating hose or tubing |0'lc.

The tank coil I 2-I3 is also iluid cooled. As shown in Fig. 3, the coil section I3 has an inner now channel |3a and a separate outer flow channel. In other words, the coil sections I2 and I3 are each constructed of double tubing or of a single tube having separate flow channels therein. Electrically the inner and outer sections are of course united. The inlet is indicated at the pipe |08, Fig. 2, and the iiuid ilow may be traced through the control valve |09, the pipe I0 which leads to a rubber or insulating hose I I I coiled on a suitable insulating support I2 located at the central part of the coil assemblies. The opposite end |||a of the insulating hose is connected to the inner fluid channel of the coil I2. The iluid iiows through the inner channel and exits by Way of pipe ||4 in which there is included a T-connection I5 which is plugged so that all :duid flow is by way of the line I IB through the valve I 1 and to the lowermost rpart or the cooling jacket IIS of one of the tubes. The fluid returns by way of the branch lines |l9, |20, valve 2| and a pipe line |22 which is located directly below the pipe line I4. The line |22 is connected to the outer now channel of the coil I2. It may here be observed the upper line |I4 is extended beyond the T-fltting ||5. This extension provides an electric connection by Way of conductor |23 to the anode of the tube cooled by the jacket |I8.

Continuing with the description of the flow of the cooling i'iuid, it flows through the outer now channel of the coil I2 and from the opposite end of the coil through pipe |24 to the inner flow channel |3a, Fig, 3, of the coil I3, from which it exits by a pipe line |25 located directly above the pipe line |25. The pipe |25 leads to the cooling jacket ||8a of the other tube. The return circuit is by way of the line |26, Fig. 2, and through the outer flow channel of coil |3, thence through a length of rubber or insulating hose interwound with the hose III. The cooling fluid or water exits from the system through the line I21,Fig.2.

While a suitable iluid circulating system has been illustrated it will be understood insofar as the electrical operation of the system is concerned, the coil assemblies may be air cooled, either by circulation of air or by radiation of heat directly into the atmosphere.

The assembly of the stationary coil |2| 3 and the pivotally mounted coils 40-4I and 45 is relatively compact and comprises a sub-assembly which is directly incorporated into the mechanical structure forming the system as a whole. If desired, the functions of the pivoted coils may be interchanged, that is, the output coil 45 may be stationary and the tank inductor coil |2-I3 mounted for pivotal movement; or coils 40-4I may be interchanged in function with coil 45. The sub-assembly of coils is shown in Fig. 3 to be mounted upon the frame F which comprises vertical channel irons -and horizontal crossbars of insulating material, preferably hardwood. The motors which operate the coils 40-4I and 45 are mounted at the base of the frame which also carries the support for the rubber or insulating tubing |I|. The axes about which these coils rotate are located so that the respective coils may be rotated or swung into and out of inductive relation with the divided tank coil |2-I3.

Again referring to Fig. 2, the connections from the feed back coils 40 and 4| are by way of conductors I0| and |02. These connect directly with an exciting coil 42 comprising a tubular conductor supported directly above the input tank'iuductor I5. Though a single turn is shown it is understood additional turns may be utilized if desired. The capacitor I 6 comprises a series of condenser plates located one above the other, the assembly as a whole being disposed directly beneath the coils or inductors I5 and 42, As best shown in Fig, 6, the coil 42 is supported by means of porcelain insulators |28, corresponding ends of which insulators are attached to coil 42 with the opposite ends thereof attached to insulating cross members |29 and |30 of a supporting frame. These cross members are carried by upright supports |3I and |32. The upright supports |3| for the cross member |29 are provided with a plurality of holes |3|a and the upright supporting members |32 for the cross member I 30 are provided with similar holes |32a. It will thus be seen that the cross members |29 and |30 together with the insulators |28 and the coil 42 form a unit which may be held in dierent predetermined positions depending upon which of the vertically spaced holes |3|a and |32a are utilized to receive the fastening bolts which extend through supporting angle irons carried by the cross members |29 and |30. As shown, the coil 42 is relatively close to the cooperating coil l5. By moving the assembly upwardly, this spacing may be increased thereby to decrease the coupling between the coils 42 and I5. The input tank coil or inductor I5 is constructed of copper pipe or tubing of substantial diameter, For example, in one embodiment of the invention the diameter of the copper pipe was between three and four inches. The inductor I5 comprises two U-shaped sections with the respective arms thereof extending toward each other. The closed ends |a and |521 extend slightly beyond the upright supports |3| and |32. The ends of the inwardly extending arms terminate in L-ttings into which there are solder-sweated depending tubular conductors |38-|4|. These depending tubular conductors are respectively supported on insulators. three of which are shown, the insulators |42-I44. The depending conductors |40I4| are interconnected by a snorting bar I 45 which by means of clamps |46 and |41 may be secured in any desired position along the lengths of conductors |40 and |4I.

Electrically the effect of moving the snorting bar |45 upwardly from the position illustrated is to decrease the inductance of the coil I5. This is desirable for initial adjustment of the oscillator as a whole. The opposite depending conductors |38 and |39 are conveniently located for short electrical connections to the tank capacitor I6. One connection, from the depending conductor |38, is completed by means of a relatively wide band or strap of copper |48. One end is electrically connected to the conductor |38 and at the opposite end is electrically connected to one set of the plates I6a of the capacitor I6. Similarly, a like strap-conductor (not shown) extends from the other depending conductor |39 to the other set of plates IBb of the capacitor I6. In addition to the support by the insulators |42-I44 at the bottom of the frame, the coil I5 is supported from lower cross bars |50 and |5I of the frame by means of insulators |52-I55. The cross bars and upright support members thus far described may be and are preferably constructed of hardwood having good insulating properties. This superstructure as a whole may be carried by metallic channel irons |56 having their lower ends interconnected by means of a rectangular metallic frame |51.

The plates |6a and IGb which form the capacitor I6 are respectively provided with four rods which carry plate-spacing members. Two of these rods, |10 and |1I, for the plates |6a, appear in Fig. 6. One of these rods, |69, for the plates IGb, may be seen in Fig. 6, as well as the ends of the two rods on the opposite side of plates |6b. These rods are carried on cross members. For example, the rods |10 and I1| are bolted to the cross member |12, itself supported on insulators |13 and |14, which in turn are fastened to the frame. Similarly, the insulators |15 and |16 support a similar cross member (not shown) through which the rod |69 and its companion rod (not shown) extend. The rods |10 and |1| extend through and are secured to an upper cross member |14a carried by insulators |66. On the opposite side of the plates |6a like rods extend through the cross member |61 carried by the insulators |65. These insulators are supported from a rectangular frame |60 carried by four bolts, two of which, the bolts |6| and|64, are shown.

The two rods, including the rod |69 for the plates |6b, extend through a cross member |68a carried. between the depending insulators |65. The two rods on the opposite side of plate |6b extend through and are fastened to the cross member |68, carried by two depending insulators, only one of which, the insulator |66a, is shown.

It will thus be seen that the capacitor I6 is of rugged construction. It may also be observed that from the frame |60 there extend upwardly supporting members for an insulating cross member |80 which also carries two insulators, |8| and |82,

10 which assist in supporting the ends of conductor 42 from which extend the conductors |0| and |02.

By utilizing the relatively large conductor to form the tank coil or inductor I5, the tank circuit including this coil and the capacitor I6 has a high electrical efficiency. Its resistance is not only low but it also has a large surface from which any heat generated therein is quickly dissipated or lost. In terms of the Q of the circuit, it may be and preferably is of the order of 1000 or more. The grid tank circuit has a high capacitance, an exceedingly low resistance and a relatively low inductance. Electrically the input or grid tank circuit may be isolated from the anode tank circuit. It is inductively coupled thereto through the adjustable feed back coil t0-4|. This grid tank circuit functions to store far more energy than is needed to control the grids of the respective tubes I0 and II, Fig. 1. By storing a relatively large amount of energy as compared with that dissipated in the control of the tubes I 0 and the circuit is stable. There is little tendency for the frequency to deviate due to reiiected impedance from the load circuit. The rectangular configuration of the tank circuit is advantageous in that relatively short leads may be used between the tank coil and the grids of the tubes. In other words, as shown in Fig, 2, the tube locations are adjacent the opposite ends of the rectangular tank circuit and the leads from the grids to the tank circuit are relatively short. Specifically, one grid may be connected to the closed end I5a and the other grid may be connected to the closed end I 5b of the coil I5 as by a suitable clamp or by a soldered or brazed connection such as are well known to those skilled in the art. It may be further observed that the adjustable taps I8 and 2| of Fig. l diagrammatically indicate adjustability of the voltage applied to the respective grids of the tubes I0 and Il. While this diagrammatic adjustment may be utilized, the shorting bar |45 of Fig. 6 performs a like electrical function and has the advantage that the input signals to the respective grids are simultaneously adjusted in magnitude depending upon the position of the shorting bar |45 with respect to the depending conductors |40 and |4I.

The neutralizing capacitors 44 and 45 of Fig. 1 and the plate circuit capacitor I4 have not been illustrated in Figs, 2 and 6 of the drawings inasmuch as they may be of conventional design. The output capacitor I4 may also be generally similar to the design of the input capacitor I6. The spacing and size of the plates will be selected to meet the voltage-capacity requirements.

Referring to the modified form of the invention as illustrated in Fig. 7, parts corresponding with those of Fig. 1 have been identied with the same reference characters. These parts perform identical functions and the description thereof need not be repeated. In accordance with Fig. '7 the feedback circuit is tuned by means of capacitors and ISI, These capacitors are of substantial size so that the closed circuit including the feedback coil 40-4I and the input coil |12 provide for the storage of a relatively large amount of electric energy, and substantially in excess 0f that required to control the grids of the tubes I0 and I|. In other respects, the system of Fig. 'I operates in the manner already described in connection with Fig. 1

In accordance with a further modification of the invention, for some applications a simplified circuit of the type shown in Fig. 8 will be satisfactory. In accordance with this system the respective grid circuits of the tubes and are conductively connected to the respective feed back coils 40 and 4|. The anode tank circuit comprising tank coil |2|3 and capacitor 4 in this instance, determines the output frequency. As before, the cross-cross capacitors 34 and 35 in combination with the grid biasing means, including the C battery, resistor 25 and the capacitor 60, prevent the generation of oscillations whenever the feed back coils 40 and 4| are moved to their neutral positions with respect to the tank coil |2--|3 A further modiiication of the invention is shown in Fig. 9. A single electric valve |99 has its anode connected to a tank circuit comprising coils 200, 20|, and a capacitor 202. A single feed back coil 40 is conductively connected through the C battery to the grid-cathode of the valve. In order to prevent oscillations when the coupling coil 40 is in the neutral position, a voltage of correct phase to neutralize the interelectrode capacity of the tube |99 is derived through capacitor 203, and fed back to the grid-cathode circuit. It will be observed the source of anode potential is applied from the choke coil 204 to a point intermediate the coils 200 and 20|. By so locating the input circuit from the anode source of supply a neutral point is established between the coils 200 and 20 The result is that the instantaneous voltage across the coil 20| is 180 out of phase, with respect to the instantaneous voltage across the coil 200. In this manner a voltage of correct phase is derived, through the capacitor 203, effectively to neutralize the grid-cathode capacity of the tube |99.

In the various modifications of the invention, the principle of rotating the feed back coils with respect to the tank coil is utilized. While this arrangement is desirable it presents mechanical problems, solutions of which have already been presented in connection with the description of the disclosures of Figs. 2-5. A further modication of an adjustable coil-supporting assembly is shown in Fig. 10. If it is desired to rotate the tank coil |2-I3, diagrammatically illustrated in Fig. l0, the supporting conductors 45a and 45h may have insulatingly supported from them a third conductor 2 0, carried by suitable insulators such as the insulator 2| l. A flexible conductor may be attached to the conductor 2|0 by means of a clamp 2| Ia, with additional circuit connectors 2|2 and 2|3 provided for the connections to the outer end portions of the respective coils |2 and |3.

A modified form of the feed back coil assembly of Fig. 5 is illustrated in Fig. 11. In accordance with this modification of the invention, the frame F has dependent legs 2|5 and 2|6, on which are mounted bearing members 2|1 and 2|8. These bearing members rotatably support a conductor 2 9, on the respective ends of which are L-ttings 220 and 22|. Into each fitting there extends the respective conductors 222 and 223, which lead directly from coils 224 and 225. These coils are of the pancake type, that is, spirally wound turns are all located in the same plane. Connection is made to the inner ends of the respective coils by means of rigid conductors 226 and 221, which terminate in L-shaped fittings 228 and 229. The conductors 222 and 226 are mechanically locked together by insulating members 230. The insulating members 23| perform the same function for the conductors 223 and 221. The respective L-fittings 228 and 229 have extending from them conductors 232 and 233. on which are mounted additional L-iittings 234 and 235, each of which is provided with a plurality of resilient contact fingers which insure intimate and low resistance electrical contact therewith. The L- shaped ttings 234 and 235 have extending from them additional conductors 236 and 231, to which the feed back conductors |03 and |04 may be connected in manner already described in connection with Figs. 2, 3 and 5. The operating member 238, for operating the respective feed back coils 224 and 225 is in this instance mounted between the bearing members 2 1 and 2 8.

Where the supporting frame F is located below the coil assembly the construction of Fig. 12 may be conveniently utilized. The stationary bearing members 240 and 24| rotatably support a relatively large cylindrical conductor 242, to which an operating lever 243 is secured. The conductor 242 has secured to its respective ends T-ttings 244 and 245. Feedback coils 246 and 241 are supported from these T-fittings by means of rigid conductors 248 and 249, which extend therefrom. The inner ends of the coils 246 and 241 are supported by conductors 248a and 249a, mounted at their lower ends in T-flttings 250 and 25|. These T-ttings are rigidly connected to conductors 252 and 253, which extend inwardly and concentrically of the conductor 242. The respective inner ends of the conductors 252 and 253 are rotatably mounted Within conductor 242 by means of the insulating bushings 254 4and 255. Coupling members 251 and 258 are each provided with a plurality of resilient contact nngers which' engage the respective ends of conductors 252 and 253 to complete an electrical connection therewith. From the couplings 251 and 258 there extend conductors 260 and 26| which may lead to feedback conductors, corresponding with the conductors |03 and |04, and |0| and |02, Fig. 2, for the return of energy to the input circuit of the oscillator.

It will be understood these alternate arrangements are equally applicable to the output coupling coil 45, or, if desired, the tank coil |2, |3 may be pivotally mounted for relative movement with respect to its associated coils 40, 4| and 45.

Before reviewing the operation of the system as a whole as applied to the heating of suitable dielectric materials, such as the bundles of plywood 41 and 48 shown in Fig. 1, a brief review of the principles involved in the heating of dielectric material will be presented. Basically, in any electric circuit the power consumed is the product of the voltage times the current times the power factor. In the form of an equation it is:

P=EI COS 0 (1) where P equals power consumed, E equals voltage, I equals current and the cosine 0 equals the power factor.

Where the circuit includes a resistor and a capacitor in series, the current is equal to the voltage divided by the impedance, or mathematically:

If the resistance is small, as it is with a substancos 9 tially pure capacitive load, it may be neglected. When neglected, the relationship simplifies t:

Inasmuch as the capacitive reactance X is expressed by the relation where w equals 21rf (f being the frequency in cycles per second), and C the capacity of the load in farads, Equation 4 may be expressed P= E22 fC' cos 0 (5) However, the capacity of a parallel plate condenser is frequently expressed:

where A is the area. of one plate in square inches, d the spacing between the plates in inches, B a. constant to convert the expression to microfarads and K is the dielectric constant of the dielectric material or load between the electrodes.

Substituting Equation 6 in 5 and multiplying numerator and denominator by d, there is obtained p: [gram/1MB cos o 7) The expression In the foregoing V equals the volume which is equal to A times d and inasmuch as the dielectric constant K times the power factor equals the loss factor L. F. the equation may be further simplified and written In other words, neglecting the resistance as shown by the simplified Equation 4, the power consumed in generating heat between the electrodes varies with the square of the voltage gradient. As the voltage is increased, the heating effect is rapidly increased. The heating eifect varies directly with the frequency, the volume of material being heated, and with the loss factor.

Equation 9 indicates that any particular heating problem may be approached in the following manner. If a material of a. certain volume is to be heated, the first consideration will be the spacing between the plates. An applied voltage as high as will be reasonably safe from the standpoint of ashover and the like ls preferred inasmuch as the heating varies with the square of the voltage. Since the higher the voltage the more rapid will be the heating of the dielectric material, the actual voltage utilized thus depends on the physical dimensions of the material to be heated. After this variable has been determined, the only other variable in Equation 9 over which the operator has any control is the frequency. In consequence, a frequency will be selected which will produce the required heating in the desired period of time. For many applications it has been found that frequencies from 1,000,000 to 10,000,000 cycles per second are satisfactory, although there is nothing critical about the frequency. It may be low, thousands of cycles per second, or high, as in megacycles per seconds.

Where the dielectric to be heated presents substantial series resistance, it will be found that certain frequencies may produce greater heating effects than frequencies above and below that frequency. This is due to the fact that the equation, as shown by Equation 3, is not linear. That is, the impedance is a complex expression, the square root of the sum of the respective squares of resistance and reactance. The square of the voltage applied to the electrodes divided by this complex expression and the quotient multiplied by the power factor gives the power consumed or used in heating the material between the electrodes. For such materials, the maximum applied voltage is first selected from the voltage gradient standpoint. Inasmuch as the resistance of the material can be measured, the equation may then be solved for the frequency which will produce the maximum heating. This solution of Equation 3 is conventional and need not be presented here.

Again referring to Fig. 1 or Fig. 7, the tuning coil 46 comprises a relatively large coil, each turn of which may consist of copper tubing of substantial diameter. 'I'his tuning coil 46 is adequate to introduce enough inductance in series with the output coil 45 to tune the output circuit to resonance. The capacitance in the output circuit is supplied by the load, the dielectric material cornprising the plywood bundles 41 and 48. The electrodes form in effect two plates of a capacitor separated by the material being heated or treated. By tuning the output circuit to resonance at the applied frequency and adjusting the coupling between coils I2, I3 and 45, the load presents to the power generator a resistance of substantially constant magnitude. In this manner, a much greater efficiency is attained, a much greater percent of the power input is converted into heat within the load illustrated as the plywood bundles 41 and 48. This provision is in contrast with prior practice in which the load consisted of a, transmission line terminated at a xed antenna or radiating terminal. The systems of Fig. l or Fig. 7 preferably operate in manner similar to class C amplifiers. That is, the bias as applied by the bias battery C, the resistor 25 and the capacitor 60 is sufiiciently negative to prevent anode or plate current flow except during a fraction of the positive half cycle of the grid excitation cycle. This means that the grid bias must be reduced by the input signal to approximately zero before the plate current can ilow. At that time an impulse of plate current flows from one or the other of the anodes of the tubes I 0 and I I to the cathode through the t lnk circuit I2-I3-I4. These current impulses through the respective anode or plate circuits of tubes I0 and II supply energy to the tank circuit I2-I3-I4 and produce oscillating current ow therein at a frequency depending upon the relative values of the inductance of coil I2-|3, and of the capacitance of the capacitor I4. There are inherent losses within the tubes themselves. The principal one is that due to the heating of the anode. Because of the heat developed at the anode it is necessary to provide the uid cooling system heretofore described. The heating of the anode is proportional to the productof the instantaneous anode current times the instantaneous anode-cathode voltage. For most eflicient operation, it would be desirable to provide for anode current ilow at a time when the anode voltage is at its minimum. While thi-s may be done, the duration of the current impulse must be reduced to such a degree that the total power output is also reduced. Therefore, the preferred operation is with a grid bias such that the anode current flows from a tube during a part of a half cycle, with resultant efficiencies of conversion of the direct current power into high frequency high voltage alternating current power of the order of sixty percent to seventy-five percent.

Inasmuch as the foregoing considerations present the requirements for operation in accordance with this invention, it will now be seen that the foregoing manner of connecting the load to the secondary circuit has many advantages. The voltage applied to the load, by reason of the transformation from the primary winding, the coil I 2-I3 to the secondary winding comprising the coil 45, may be continuously varied. That voltage may be either greater or less than across the tank condenser I4. By placing the load in the secondary circuit, varying values of resistance may be reflected into the primary circuit. This makes possible the following procedure in loading the power amplifier.

With the respective coils in approximately the positions shown in Fig. 3 and the circuits otherwise energized as indicated in Fig. 1, the motor controlling the feed back coil 4ll-4l is energized in a direction to move this coil toward the output coil I2-I3. A small voltage is thereby induced into the feed back coil 40-4l which of course is transmitted to the coils 42 and 43 at the input side of the amplifier or oscillator. The motor 11, Fig. 3, is then energized in a direction to move the output coil 45 toward the tank coil l2|3. While in a position of loose coupling the secondary circuit including the coil 45 is tuned by the inductor 46 to resonance. Resonance may be obtained by measuring the voltage across the load, that is, between the center platen 5l of the press and ground G. Upon varying the tuning coil or inductor 46, this voltage will rise to a maximum, indicative of a resonant condition. When this is done, the output coil 45 is moved to a position of greater coupling with respect to the coils |2-I3. This has the effect of decreasing the voltage across the coils I 2-l3 and of increasing the direct current input from the B supply. The feed back coil dll-4l is then moved to a position of greater coupling with respect to the coil l 2-I 3, and the greater excitation applied to the input tank circuit l5-l6 and to the grids of the tubes produces a rise in the voltage across the output tank coil |2-I3, and a rise in the current through the coils. This in turn increases the voltage across the load. The operation is then repeated. The output coil 45 is moved to a position of still greater coupling, followed by adjustment of the feed back coil 40-4l to a position of greater coupling. By measuring the direct current flowing to the circuit from the source of anode supply and the voltage developed across the output coil I2-l3 a direct indication is obtained as to the power output. When these two quantities reach rpredetermined values as determined by the voltage and current load characteristics oi the tubes utilized, the operator knows that the oscillator has reached full load, and it is operating with good plate emciency.

. It is to be observed that the foregoing adjustments are made during operation of the oscillator or high-frequency generator. In similar manner, adjustments may be made at any time during the operation of the oscillator. In this way, adjustment of the applied load may be made at any time depending upon the heating requirements as imposed by the character of the material which is being heated.

While embodiments of the invention have been described, it will be understood that further modications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a high-frequency electrical generator of the push-pull type having a pair of valves each provided with an anode, a cathode, and a control grid, the combination of an output tank circuit connected to said anodes, and an input tank circuit connected to said control grids and including a coil formed by a conductor of large cross-sectional area and 0f exceedingly low resistance at the operating frequency, said coil having a generally rectangular shape and at its midportion having two pairs of depending legs, an adjustable shorting bar connected across an adjacent pair of said legs, a capacitor disposed between said two pairs of depending legs, one set of plates thereof being electrically connected to one of the other of said pairs of legs and the other set of plates being electrically connected to the remaining one of said legs, an exciting coil of generally like rectangular construction, means including electrical insulators for supporting said exciting coil adjacent to and in inductive relation with said first-named coil, an electrical circuit including said exciting coil and a second coil inductively. coupled to said output tank circuit, and means connecting said control grids to separated points on said first-named coil.

2. In a high-frequency generator including at least one electric valve, the combination of an output tank circuit connected to said valve, an input tank circuit connected to said valve and comprising an inductor formed by a conductor of large circumferential area and having a double U-shape, the respective legs of each U-shaped portion extending towards each other and in the same plane, the four ends of the adjacent legs thereof having depending end portions, an adjustable conductor interconnecting two of said depending end portions, a capacitor comprising two sets of plates disposed adjacent said inductor, and means connected one of the remaining depending end portions to one set of plates, and the last of said depending end portions to the other of said sets of plates, means for applying to said input tank circuit energy from said output tank circuit comprising a generally rectangular shaped coil overlying said inductor and inductively coupled therewith, and means for supporting said coil in selected fixed relation therewith.

3. In a high-frequency generator, an input tank circuit comprising a capacitor having two -sets of plates spaced one from the other, a pair of upstanding conductors, means electrically connecting one set of plates to one of said conductors, and the other set of plates to the other of said conductors, said upstanding conductors being in a region spaced above said capacitor and extending outwardly from each other and back again towards each other to form a single turn coil, said conductors in the region where they again approach eachother having depending end portions, and an adjustable snorting bar slidable casacca 17 with respect to said depending end portions and electrically connecting them together.

4. An oscillator of the push-pull type comprising a pair of electric valves each provided with an anode, a cathode, and a control grid, an output tank circuit connected to said anodes and including a divided coil forming the inductor thereof, a capacitor in shunt with said inductor for tuning the same to resonance at a desired output frequency, a pair of coils each having ends which extend away from the peripheries thereof, separate conductive means electrically insulated from each other and rigidly connected to the respective ends of said pair of coils forming a pivotal mounting for each of said coils, and `Ymeans for moving each of said coilsbetween positin'so' minimum and maximum coupling with respectto said divided coilY 5. In an oscillator, a coil formed of copper tubing, rigid conductive tubes connected to the respective ends of said coil and forming the support therefor, means including one of said rigid tubes forming a pivotal support for said coil, and means connected to said one tube for adjusting the position of said coil.

6. In a high-frequency system having thermionic tubes, the combination of a cooling system which comprises an inductance coil formed in two sections and electrically connected together, each section of said coil having a double now passage therethrough, means including uid connections for introducing cooling uid through a ow passage of one coil and for circulating it around the anode of one of said tubes, returning it through the other now passage of said one section, thence through a ow passage of the other section, and around the anode of another tube, with a return path through the other flow passage of said other section.

7. In an oscillator, a coil formed of a rigid electrical conductor, rigid supporting conductors respectively connected to the ends of said coil, means for pivotally mounting said coil from the ends of said supporting conductors comprising a bearing extending at right angles to said supporting conductors to provide pivotal support for one of them and an electrical connection thereto, and means mechanically supporting said other supporting conductor and providing an electrical connection thereto.

8. In an oscillator, a divided coil comprising at least two coil sections, rigid supporting conductors mechanically and electrically connected to the respective ends of said coil sections and forming the sole support therefor, pivotal means mechanically and electrically connecting two of said supporting conductors, and electrically separate means mechanically and electrically connected to the remaining conductors to assist in the support of said coil sections without interfer- 60 ing with pivotal movement of said coil about said pivotal means.

9. In an oscillator of the p ulljtypehaving a tubular inductor common to tiescieted in push-pull circuit relation, the combination of means for cooling the anodes thereof, which comprises means for introducing a, cling uidat the electrical center of said inducto`rfh`eanjpr conducting said nuuiiirngnslresaafinw passageffhalf of Y. saidwllldlglgtglnlf Ylint() cententhereoi., meansforpassingthe t hro i175 18 a flow passage of the other half of said inductor into cooling relation with the anode of the other of said tubes, means for returning the uid through a separate ow passage of said other half of said inductor to the electrical center thereof,

and means for producing forced flow of said iluid through said coil and in cooling relation with said anodes.

10. An inductance system for a self-excited oscillation generator comprising a. tank circuit inductor formed in two sections spaced one from the other, each section having a double fluid flow path, means connecting one path of one section in series with one path of the other section, said means also connecting said two sections to forni a single electrical path between them, a pair of coils movable into and out of inductive relation with said inductor, means electrically interconnecting said pair of coils and also forming a 20 pivotal mounting for movement thereof about an i uctor. "./111 An inductance system for a self-excited f oscillation generator comprising a tank circuit inductor formed in two sections spaced one from the other, each section having a double fluid flow path, means connecting one path of one section in series with one path of the other section, said means also connecting said two sections to form a single electrical path therebetween, a pair of coils movable into and out of inductive relation with said inductor, means electrically interconnecting said pair of coils and also forming a pivotal mounting for movement thereof about an axis removed from the periphery thereof, an out- 5 put coil formed of a Iuid conduit which is also an electrical conductor, supporting means connected to each end of said output coil for pivotally mounting it for movement into and out of inductive relation between said two sections.

means for circulating cooling fluid through said supporting means and the turns of said output coil, means for independently adjusting the positions of said pair of coils and of said output coil with respect to said inductor, and an inductance 5 coil of helical shape connected in uid and electrical conducting series circuit relation with said output coil.

12. In a high-frequency heating system including an oscillator, an output coupling coil formed of a rigid electrical conductor, rigid supporting conductors respectively connected to the ends of said coil, means for pivotally mounting said coil from the end portion of at least one of said supporting conductors to provide pivotal support 05 therefor, said means comprising a bearing extending at right angles to said supporting conductors, means mechanically and rotatably supporting said other supporting conductor and providing an electrical connection thereto, and means mechano ically'connected to one oi said rigid conductors for adjusting the position of said coil about its said pivotal support.

PAUL D. ZO'I'I'U.

(References on following page) 19 REFERENCES CITED Number UNITED STATES PATENTS Name Date Senseney Oct. 24, 1893 Gebhard Apr. 8, 1930 Finch et a1. Feb. 13, 1934 Lynn Dec. 4, 1934 Conklin Jan. 1, 1935 Pettengill Oct. 22, 1935 Eitel Jan. 7, 1936 Number 10 N umber Name Date Crosby July 28, 1936 Webster Jan. 18, 1938 Chaee Feb. 21, 1939 Williams May 30, 1939 Davis et a1 Oct. 15, 1940 Worden et al. Apr. 22, 1941 FOREIGN PATENTS Country Date Sweden May 17, 1926 Great Britain Aug, 12, 1920 

